4.3 Phonetic Segments and Features
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In our thinking about speech sounds so far, we’ve focused almost entirely on segments. Segments are the individual speech sounds, each of which gets transcribed with an individual symbol in the IPA. We’ve seen that any given segment can influence the segments that come before and after it, through coarticulation and other articulatory processes. And we’ve also seen that segments can be grouped together into syllables, which we look at in more detail in another unit. Within the grammar of any language, two different segments might contrast with each other or might not.
So we’ve been talking as if segments are the smallest unit in speech, but in fact, each speech segment is made up of smaller components called features. Each feature is an element of a sound that we can control independently. To see how features work, let’s look at a couple of examples. We can describe the segment [b], for example, as being made up of this set of features. First, [b] is a consonant (meaning it has some obstruction in the vocal tract), so it gets the feature consonant indicated with a plus sign to show that the consonant feature is present. Looking at the next feature, sonorant, notice that it’s indicated with a minus sign, meaning that [b] is not a sonorant. The feature sonorant, of course, has to do with sonority. We know that stops have very low sonority because the vocal tract is completely closed for stops, so stops are all coded as [-sonorant]. The next feature, syllabic, tells us whether a given segment is the nucleus of a syllable or not. Remember that the most common segments that serve as the nucleus of a syllable are vowels, but stops certainly cannot be the nucleus, so [b] gets labelled as [-syllabic]. These first three features, consonant, sonorant, and syllabic allow us to group all speech segments into the major classes of consonants, vowels, and glides. We’ll see how in a couple of minutes.
This next set of features has to do with the manner of articulation. The feature continuant tells us how long a sound goes on. Stops are very short sounds; they last for only a brief moment, so [b] gets a minus sign for continuant. We also know that [b] is not made by passing air through the nasal cavity, so it also gets a minus sign for the feature nasal. And [b] is a voiced sound, made with vocal folds vibrating, so it is [+voice].
The last feature we list for [b] is [LABIAL] because it’s made with the lips. (Stay tuned for an explanation of why some features are listed in lower-case and some in upper-case.)
This whole list of features is called a feature matrix; it’s the list of the individual features that describe the segment [b], in quite a lot of detail! Because features are at the phonetic level of representation, we use square brackets when we list them. You often see a feature matrix listed with a large pair of square brackets, like this, but we’ll just use individual square brackets on each feature.
Now I want you to notice something. If we take this whole feature matrix and change the value of just one feature, changing the feature voice from plus to minus, now we’re describing a different segment, [p]: [p] has every feature in common with [b] except for voicing. Likewise, if we take the feature matrix for [b] and change the value of the feature continuant from minus to plus, now we’re describing the segment [v], which has all the same features as [b] except that it can continue for a long time because it’s a fricative. Or if we take the feature matrix for [b] and change the feature nasal from minus to plus, this has the effect of also changing the sonorant feature to plus as well, because circulating air through the nasal cavity adds sonority. Now, this feature matrix describes the properties of the segment [m].
So each feature is something that we can control independently of the others with our articulators. And changing just one feature is enough to change the properties of a segment. That change might lead to a phonemic contrast within the mental grammar of a language, or it might just result in an allophone of the same phoneme.
It turns out that segments that have a lot of features in common tend to behave the same way within the mental grammar of a language. And we can use these features to group segments into natural classes that capture some of these similarities in their behaviour.
Let’s look again at the feature matrix for [b]. If we take away the feature that describes its place of articulation, we end up with a smaller list of features. This smaller list describes not just a single segment, but a class of segments: all the voiced stops. By not mentioning the place feature, we’ve allowed this matrix to include segments from any place of articulation, as long as they share all these other features. These three segments have all these features in common: they’re a natural class. If we remove another feature, the voicing feature, the natural class gets bigger: now we’ve got a feature matrix that describes all the stops in English, including those that are [+voice] and those that are [-voice]. So you can see that this system of features is very powerful for describing classes of segments that have things in common. We’ll learn more about natural classes in the next unit.